could life itself have made the planet uninhabitable?
Four billion years ago, the solar system was still young. Almost fully formed, its planets were beginning to suffer asteroid strikes a little less frequently. Our own planet could have become habitable 3.9 billion years ago, but its primitive biosphere was very different from what it is today. Life had not yet invented photosynthesis which, some 500 million years later, would become its main source of energy. The primordial microbes – the common ancestors of all current life forms on Earth – in our planet’s oceans therefore had to survive on another source of energy. They consumed chemicals released from within the planet by its hydrothermal systems and volcanoes, which accumulated as gases in the atmosphere.
Some of the oldest forms of life in our biosphere were microorganisms called “hydrogenotrophic methanogens” which particularly benefited from the atmospheric composition of the time. Feed on CO2 (carbon dioxide) and H2 (dihydrogen) which was abundant in the atmosphere (with H2 representing between 0.01 and 0.1% of atmospheric composition, compared to about 0.00005% today), they captured enough energy to colonize the surface of our planet’s oceans.
In return, they release large quantities of CH into the atmosphere.4 (aka methane, hence their name), a potent greenhouse gas that has been building up and warming the climate. Since our sun at the time was not as bright as it is today, it might not have been able to sustain temperate conditions on the planet’s surface without the involvement of other aspects. Thus, thanks to these methanogens, the very emergence of life on Earth may itself have contributed to ensuring the habitability of our planet, setting up the conditions conducive to the evolution and complexification of the biosphere. earth for the billions of years that followed.
If this is the most likely explanation for the early development of habitability on Earth, what about other planets in the solar system, like our neighbor the Red Planet? As we continue to explore Mars, it becomes increasingly clear that similar environmental conditions were developing on its surface at the same time as those that allowed methanogens to thrive in Earth’s oceans.
Microbial life may have resided within the top four kilometers of the porous crust of Mars. There it would have been safe from harsh surface conditions (especially harmful UV rays), more favorable temperatures compatible with liquid water, and a potentially abundant source of energy in the form of atmospheric gases released within the crust.
In light of these aspects, our research group was naturally led to a key question: did the same life-generating events that happened on Earth also happen on Mars?
A portrait of Mars from four billion years ago
We attempted to answer this question using three models, which led to the results recently published in the natural astronomy scientific journal. The first model allowed us to estimate how the volcanism on the surface of Mars, the internal chemistry of its atmosphere and the emission of certain chemicals into space could have determined the pressure and the composition of the atmosphere of the planet. The same characteristics would then have determined the nature of the climate.
The second model sought to identify the physical and chemical characteristics of the porous crust of Mars, namely the temperature, the chemical composition and the presence of liquid water. These were partly determined by surface conditions (i.e. surface temperature and atmospheric composition) and partly by the internal characteristics of the planet (i.e. thermal gradient internal and the porosity of the crust).
These first two models allowed us to simulate the superficial and underground environments of the young planet Mars. However, many uncertainties remained regarding the main characteristics of this environment (for example, level of volcanism at the time and thermal gradient of the crust). To address this issue, we used our model to explore a large number of potential features, resulting in a set of scenarios for how Mars looked about four billion years ago.
The third and final model concerns the biology of hypothetical Martian methanogenic microorganisms, based on the theory that they would have been similar to methanogens on Earth, at least in terms of energy requirements. Using this model, we were able to assess the habitability of the conditions on Earth for our microbes compared to the environmental conditions underground on Mars, according to each environmental scenario generated by the two previous models.
When the given conditions were deemed habitable, the third model assessed how these microorganisms would have survived below the surface of Mars and – alongside the crustal and surface models – how this subterranean microbial biosphere would have influenced the chemical composition of the crust, as well as the atmosphere and the climate. By combining the microscopic scale of the biology of methanogenic microbes with the global scale of the climate of Mars, these three models together made it possible to simulate the behavior of the Martian planetary ecosystem.
Subterranean habitability most likely existed in the crust of Mars
A number of geological clues point to a flow of liquid water on the surface of Mars four billion years ago, which would have formed rivers, lakes and, possibly, even oceans. The Martian climate was therefore more temperate than it is today. In explaining how such a climate could have occurred, our surface model assumes that Mars had a dense atmosphere (about the same density as our own planet today) that was particularly rich in CO2 and H2even more than planet Earth at the time.
This CO2-the rich atmospheric context may have essentially provided the atmospheric H2 with the characteristics of a remarkably potent greenhouse gas. This H2 would have been even more powerful than CH4 under the same conditions. In other words, if 1% of the Martian atmosphere had been H2the climate would have been warmer than if 1% had been CH4.
According to several of our scenarios generated by our models, this greenhouse effect alone would not have been enough to produce the climatic conditions necessary to maintain liquid water on the surface of Mars, which means that the red planet was covered of ice. Moreover, if there had been appropriate temperatures deep within the Martian crust, they would not have made it more habitable either. Blocked by surface ice, no atmospheric CO2 and H2 – the essential energy source for methanogenic life – could have penetrated the crust.
Nevertheless, most of our scenarios indicate that the presence of liquid water on the surface of the planet would have been possible at least in its hottest regions, where atmospheric CO2 and H2 could indeed have penetrated the crust. Our biological model attests that in all these scenarios, the methanogenic microorganisms would have found suitable temperatures and would have had access to a sufficiently large source of energy for their survival in the first hundred meters of crust. In short, although we do not yet have factual proof of life on Mars, past or present, the Martian crust very probably hosted four billion years ago an underground biosphere composed of methanogenic microorganisms.
An ice age triggered by a primitive biosphere
Could these hypothetical Martian methanogenic life forms have warmed their planet’s climate in the same way as their Earth counterparts? Alas, the answer seems to be: no. An underground methanogen-based biosphere would have consumed the vast majority of the planet’s H2 and released considerable amounts of CH4leading to profound changes in the Martian atmosphere.
However, as we have seen, H2 was a more potent greenhouse gas than CH4 in the context of the early Martian atmosphere, their respective greenhouse effects being opposite to those observed in present-day Earth’s atmosphere, or what would have been observed in the early Earth’s atmosphere. While the emergence of methanogenesis on Earth helped establish a favorable climate and consolidate terrestrial habitability, methanogenic life on Mars – by consuming most of the planet’s atmospheric H2 – would have considerably cooled its climate by several tens of degrees and contributed to greater ice cover. Even in regions without surface ice, our hypothetical microorganisms would likely have sought more viable temperatures, moving deeper into the crust and further from their atmospheric energy source. In this way, the actions of these lifeforms would have made Mars less hospitable to life than it initially was.
Self-destruction: a norm for life in the universe
In the 1970s, James Lovelock and Lynn Margulis developed the Gaia hypothesis, which proposes that Earth’s habitability is maintained by a synergistic, self-regulating system involving both the terrestrial biosphere and the planet itself. We, the human species, are an unfortunate anomaly in this theory. The Gaia hypothesis has since prompted the emergence of the idea of the “Gaian bottleneck”. This posits that the universe does not lack the necessary conditions for life, but that when life does appear it is rarely able to maintain the long-term habitability of its planetary environment.
The conclusions of our study are even more pessimistic. As the example of Martian methanogenesis shows, even the simplest life forms can actively jeopardize the habitability of their planetary environment.
Boris Sauterey, Researcher in evolutionary ecology, marine ecology, and astrobiology, École Normale Supérieure (ENS) – PSL
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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